1. Field of the Invention
The present invention relates to an electronic device and a method for fabricating an electronic device, and more particularly relates to a thermal infrared sensor and a thermal infrared image sensor.
2. Description of the Related Art
An infrared sensor for detecting an electromagnetic wave (or an infrared ray) with a wavelength of 3 μm to 10 μm has been used as a heat sensing sensor in crime prevention, measuring, remote sensing and various other fields of applications. An infrared image sensor, in which such sensors are arranged as a two-dimensional array, can obtain an even greater amount of information as a thermal image, and has been used extensively in those fields of applications.
Infrared sensors are roughly classified into quantum sensors and thermal sensors. A quantum sensor is a sensor that is made of compound semiconductors and that operates by utilizing the band-to-band transition. Such a quantum sensor has higher sensitivity and higher response speed than a thermal sensor but operates at relatively low temperatures, thus requiring a cooling mechanism. That is why it is difficult to reduce the size or manufacturing cost of such a quantum sensor and it is not easy to apply it to cars, crime prevention tools and various other consumer electronic products.
On the other hand, a thermal sensor has lower sensitivity than a quantum sensor but needs no cooling mechanism. For that reason, it is relatively easy to reduce the size and price of such a sensor, and therefore, it has been used extensively in various consumer electronic products. The thermal sensors include thermopile types, bolometer types and pyroelectric types.
A thermopile type includes a portion in which a lot of thermocouples are connected in series together as a thermal sensing portion. The thermal sensor may includes a resistor that is made of a material, of which the electrical resistance has significant temperature dependence. By detecting a variation in the amount of current flowing through that resistor, the thermal sensor can measure the temperature. Meanwhile, a pyroelectric type detects charge to be produced on the surface of a tourmaline crystal, for example, as the temperature varies, thereby sensing the temperature variation.
A thermal sensor of any of these types has a heat insulation structure to prevent the heat from escaping from its infrared sensing portion, thereby maintaining the sensitivity of the sensor reasonably high. An exemplary heat insulation structure for such an infrared sensor is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-106896 (hereinafter “Patent Document No. 1”), for example.
Hereinafter, the structure of a thermal infrared sensor as disclosed in Patent Document No. 1 will be described with reference to FIG. 27, in which FIG. 27(b) is a plan view illustrating main portions of this infrared sensor and FIG. 27(a) is a cross-sectional view of the sensor as viewed on the plane 27b-27b. 
The infrared sensor shown in FIG. 27 includes a substrate 240 of silicon, for example, and a photosensitive section 241 that is supported on the substrate 240. The photosensitive section 241 includes a bolometer portion 242, of which the electrical resistance has temperature dependence, and wiring 243 for measuring the electrical resistance of the bolometer portion 242. And the photosensitive section 241 functions as a heat sensing section for the infrared sensor.
On the upper surface of the substrate 240 that is opposed to the bolometer portion 242, a recess has been cut so as to leave a gap between the photosensitive section 241 and the substrate 240. Such a recess may be formed by selectively removing a predetermined region of the substrate 240 by either a wet etching process or a dry etching process.
The photosensitive section 241 contacts with the substrate 240 at contact portions 245. Both ends 244 of the wiring 243 extend over the contact portions 245 and are connected to a read circuit (not shown).
Hereinafter, it will be described how the infrared sensor shown in FIG. 27 operates.
When the photosensitive section 241 absorbs an infrared ray, the temperature at the bolometer portion 242 rises. As a result of the rise in temperature, the resistance of the bolometer portion 242 changes. In such a state, current is supplied to the bolometer portion 242 through the wiring 243 and a variation in voltage, caused by the change of resistance, is detected. And based on the magnitude of this voltage variation, the energy of the infrared ray that has been incident on the photosensitive section 241 can be calculated.
The photosensitive section 241 preferably has a structure that can prevent the thermal energy, produced upon the exposure to the infrared ray, from escaping to the outside. In the example illustrated in FIG. 27, the area of contact between the body of the photosensitive section 241 and the substrate 240 is minimized to increase the heat insulation property. Also, the portions including both ends 244 of the wiring 243 are elongated portions extending from the body of the photosensitive section 241 to reduce the conduction of the heat to the substrate 240.
As can be seen, an infrared sensor is required to further increase its temperature in response to an incoming infrared ray, and eventually exhibit higher infrared sensitivity, by improving its heat insulation property.
Meanwhile, an infrared sensor, in which electrical switches such as transistors are arranged between the photosensitive section and an infrared detector circuit, is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-148111 (hereinafter “Patent Document No. 2”), for example. In the infrared sensor disclosed in Patent Document No. 2, a plurality of pixels that are arranged as a two-dimensional array (will be referred to herein as a “heat sensing section”) and horizontal and vertical scanning circuits and other circuits for performing infrared image sensing by driving these pixels (which will be referred to herein as a “detector circuit section”) are integrated together on the same semiconductor substrate.
In such an infrared sensor, electrical switches for sequentially selecting one of those pixels of the heat sensing section after another are arranged on the semiconductor substrate and the heat sensing section and the detector circuit section are electrically connected or disconnected to/from each other by opening and closing those electrical switches.
In the infrared sensor shown in FIG. 27, the contact portions 245 are interposed between the photosensitive section 241 and the substrate 240, and therefore, it is difficult to prevent a lot of heat from escaping from the photosensitive section 241 toward the substrate 240 by way of these contact portions 245. Nevertheless, if the contact portions 245 had an even smaller size, then the resultant rigidity would be too low to support the photosensitive section 245 and the sensor could be broken more easily.
Meanwhile, in the infrared sensor disclosed in Patent Document No. 2, the heat sensing section and the detector circuit section are electrically connected or disconnected by turning the electrical switches. However, since this switching is done electrically, the heat sensing section and the detector circuit section are always connected together in terms of heat conduction. That is to say, those electrical switches cannot interrupt the transfer of heat between the heat sensing section and the detector circuit section.
In order to overcome the problems described above, an object of the present invention is to provide an electronic device with improved heat insulation properties.